This invention relates to a double-metal-coated metal sulfide powder consisting of metal sulfide particles each having a platinum group metal coating layer and an outer coating layer of a metal or an alloy which contains at least one of iron, copper, nickel, and cobalt, and a process of producing such powder.
The composite powder of a double-coating type thus obtained is exceedingly desirable as a solid lubricant for use in the self-lubricating powder materials to be shaped and sintered to oilless bearings, sliding parts, and the like.
The self-lubricating parts such as those of oilless bearings have usually been fabricated by mixing the powder of copper, tin or other metals as the base with the powder of a solid lubricant, typified by molybdenum disulfide, pressing the mixture in dies to produce compressed shapes, sintering the shapes, and finally impregnating the products with oil. However, if such a solid lubricating powder of molybdenum disulfide or the like is directly added to the base metal powder, a good self-lubricating product will not result because of ununiformity in mixing and low strength attained on sintering. For this reason attempts have been made to use the powder of solid lubricant in the form of a composite powder consisting of the solid lubricant particles thinly coated with copper, nickel, iron, cobalt, or other second metal.
Methods of producing such composite powders have heretofore depended on thermal decomposition, reduction, vapor-phase reaction, plating reaction, and vacuum evaporation. However, they have varied disadvantages in the process of manufacture, quality of the product, and in respect of the equipment required. No method has been established yet which will produce a composite powder of a high, stable quality in a simplified process on an industrial basis. In view of these, the present applicant previously proposed a new process which could replace the conventional processes for manufacturing such composite powders with copper outer layer. The process was characterized by the steps of adding and mixing the powder of a metal or alloy baser than copper with the powder of a metal sulfide, and adding an acidic solution containing copper ions to the mixture with stirring, whereby the metal sulfide particles are coated with the metallic copper formed in a cementation reaction. Composite powders could then be obtained in a far more simplified way than before.
Although that cementation process rendered it possible to manufacture composite powders of very high quality, further investigations were made to improve the process so that easier and more uniform deposition of the copper coat with good throwing power on metal sulfide particles could be ensured. The surface of metal sulfide particles originally are not highly active and it follows that the deposition of copper or other metal on the particle surface would be facilitated if the latter could be made more active. With this in view, extensive studies have been made and, as a result, it has now been found very useful to produce a platinum group metal coating layer on metal sulfide particles.
Coating a metal sulfide surface with a platinum group metal layer may be accomplished in a number of ways. For electroless plating with platinum group metals, a variety of compositions in the form of plating solutions or the like have been proposed since many years ago. They include, for example, (a) a plating solution of a mixture containing a platinum group metal in an organic solvent, which utilizes thermal decomposition at temperatures between 300.degree. and 500.degree. C., (b) a plating paste obtained by dispersing a platinum group metal powder in a liquid containing polyterpene resin that ranges in molecular weight from 350 to 870, (c) a plating solution prepared by adding a chloride of platinum metal to a polar organic solvent or a hydrochloric acid solution, and (d) a plating mixture containing a platinum group metal compound and an organic amine as principal ingredients. Many other plating compositions have thus far been proposed or put in use.
Those compositions of the prior art have one drawback or another, such as inadequate adhesion of the plating film, too high thermal decomposition temperature, or the necessity of employing an organic solvent. In view of this, a new platinum group metal plating composition has recently been developed by the present applicant. The composition, described in Japanese patent application No. 125846/1975, may be briefly defined as one which uses at least one platinum group metal compound containing a radical selected from the class consisting of NO, NO.sub.2, NO.sub.3, and NOCl. The development of the plating composition was based on the discovery that, with platinum group metal salts containing such a radical, the thermal decomposition reaction will explosively proceed in the low temperature range of 210.degree.-300.degree. C. and, moreover, the reaction will be exothermic. As an accelerator for the thermal decomposition reaction, at least one member of the group consisting of ammonium hydroxide, ammonium nitrate, and ammonium nitrite is desirably added to the composition. In using, water is added to the composition to prepare an aqueous solution or paste as desired. The plating composition taught by the patent application No. 125846/1975 permits the use of a lower thermal decomposition temperature than are usually required, and gives a plating layer highly adherent to the surface without contaminating the working environment.
In forming the intermediate platinum group metal layer in accordance with the present invention, the technique revealed in the above-mentioned patent application was found particularly useful, although the varied electrolytic and electroless plating techniques in common use may, of course, be employed. The platinum group metal plating composition in the above-mentioned application was originally intended for forming a plating layer that represents the final surface of the object; it was not contemplated to coat the plated surface with still another metal. In the present invention, by contrast, the platinum-metal layer is intended to serve as an interposed layer between each metal sulfide particle and its final metal layer. In order to form a platinum group metal layer optimum as the interposed one, it is necessary to take into consideration the adhesion between the platinum group metal layer and the overlying metal layer, uniformity of nuclear formation and growth in the top-coat metal, and other factors. Also, because the employment of the aforementioned cementation process for the application of the top-coat metal is contemplated in the present invention, the shifting from the intermediate to the final layer formation operation is required to be made as smoothly as possible. Investigations with due consideration of these have led to a conclusion that, of the techniques available for the intermediate-layer formation, the method of patent application No. 125846/1975 is the most desirable.